She wants to make her husband her slave. This is a common wish. I will assume that her husband has no idea that he intends to become her mistress. You must start by deciding if he is submissive by nature. The next time you have sex, put him in submissive situations, see how he responds. Tell him to kiss your feet. She smacks her butt. If he seems excited, she wears pantyhose to tie him up. Tell him to call you Mistress. Make it give you pleasure. Punish him if he falls short. Now you will know for sure if her husband is submissive. If so, every time you make love, you should similarly take charge, but always up the ante. If she continues to enjoy it, then you are ready to become her Mistress.
To make her husband her slave, she must now extend the mistress-slave dynamic beyond the bedroom and into her everyday life. Before her husband leaves for work, arouse him sexually. Give him your underwear so he can think about you all day. Give him a list of instructions that he needs to do by the time he gets home. Tell him that if he doesn’t do them well you won’t let him have sexual gratification. Not allowing her to orgasm is the easiest way for a new Mistress to easily control her new slave. She will make him extremely lustful and willing to obey all your commands.
Continue these “games” until you feel the time is right to have a serious talk with him about your relationship. He will have come to himself and to you that he is submissive; also, you will have shown that you are dominant. He will also know that you enjoy the new arrangement because he will be having fun.
It is very likely that he is now aware of his own submissive side. Confront him and tell him, don’t ask, that you want to change your relationship. You want him to become your slave. At this point, it is almost certain that he will say yes, but he has done it in such a way that he gave her a choice in the matter. Then he can decide if he wants to become his slave. Nine times out of ten, the husband will say he does.